Oh @kewpiedoes I am so sorry for you, and for your poor little girl. I don't know if she calls you Mum, but that is what you are to her, and by the sounds of it, a damn good one too 🩷
I know that you say her biological mother has never been in her life, but that does sound like she is still alive, so I am wondering whether she might be serving a long sentence in prison, or in some sort of institution, I am not expecting you to answer that, as it is none of my business, and it could be more "outing" for you.
However, if there is something that could be known about, or just found out about, by her peers, she could have been asked very awkward questions, or even be being taunted by some of her peers - especially if at least one of their characters is like that of one of the few nasty people on here who have said something like "throw her out", or "call the police on her". I really hope that those few nasty responders are not actually mothers themselves, and definitely not step mothers, as I would really fear for their children's mental health 🥺
For some reason your DD lashed out at you today, which is totally outside of her normal, natural, behaviour, and to me she is directing her - newly developed teenage - anger (which I believe is totally different to a toddlers, or prepubescent child's anger) at you, probably because you are the only adult in her vicinity at the moment that she knows - or very strongly hopes - loves her unconditionally. That could quite understandably mean that you and her dad are the only adults she feels able to trust enough to be able to let herself have her very first adult type of impotent.
If that is the case, and whether her anger was caused by her newly emerging hormones, or some awful bullying, or even - God forbid - some sort of physical assault while she was out, she must have really scared, and possibly embarrassed herself, and now she is trying to understand these new and horrible emotions, and to learn how to cope with them, in a safe and reasonable way.
I am really so glad that you are you OP, and not one of those people - like the few PPs on this thread - who have shown their own lack of mature development! Maybe you could text, or even slip a note under your DD's bedroom door - which might seem more personal and caring than just a normal text.
You could tell her again, that she is not in trouble, that you love her so much, and that you understand that something will have made her behave such a way. You could also tell her, that if she doesn't even know herself why she behaved like that, it could have easily been caused by her new rampant evolving hormones that are very unfairly, usually far stronger and confusing than the ones the male of the species are subjected to! I don't know if it is still the case, but when I was becoming a 'pain in the neck' hormonal, female, teenager, we were told that the fluctuations (sp?) of a woman's hormones have been enough in France, to at, least occassionally, get a woman who had killed her cheating husband, off a long prison sentence, or maybe even having to face the administrations of "Madame Guillotine". They called such crimes, a crime of passion.
If you think that your DD could take a light-hearted joke at this time, you could add that you hope your DD is never so angry with you or her dad, that she needs to try to use the excuse of a "crime of passion" to be let off killing either/or both of you, when you were both refusing to let her go out with her friends the night before her first GCSE exam! Unless you do know for certain that she could take such a joke as light relief to her turmoil, then I guess you had better not actually add it into your conservation...
Back to the seriousness of your DD's poor plight; if neither you, or your dear partner, can gently ease out of her an explanation about any problem she is having to deal with, and if she continues to have such reactions, and/or, gives the impression during normal family times, that she is still not happy, then you and her Ddad should probably ask her GP for advice, and maybe for a referral to an appropriate and experienced child therapist.
Sorry, one last thing OP, my first impression on reading your OP, was that someone had been saying nasty things about her biological mother, or maybe about you, blaming you, her very DMum, for not having her Biological Mum in her life. Of course, I can't really have any idea about what has upset your DD, but I do hope and pray (if you don't mind), that you and her DDad, can find out quickly and gently, what the matter is, and that you can all go forward into a happy and rewarding future. xxx